The present invention relates to over-voltage protection circuits. More particularly, the present invention relates to over-voltage protection circuits having metal oxide varistor (MOV) devices as the over-voltage sensing component. Even more particularly, the present invention relates to over-voltage protection circuits having MOV devices as the over-voltage sensing component for utilization in alternating current (AC) electric power distribution.
The current state of the art electronic components (e.g., both commercial and residential computers and related equipment) accommodate transient voltage surge suppression features for protecting their highly sensitive circuits from over-voltage damage. Most transient normal use and cannot handle a major over-voltage condition as would occur during loss of neutral, loss of ground, or repetitive current pulses (e.g., from a lightning strike). Typical voltage surge protection devices comprise a plurality of fault fuses where the input is disconnected from the output as a result of an over-current condition resulting from the over-voltage. The related art surge suppression systems may also involve various metal oxide varistor (MOV) and fuse combinations. MOVs are typically non-linear devices formed from composite ceramic materials (e.g., ZnO grains in combination with an amorphous material). MOVs maintain the voltage within a narrow band (i.e., varistor voltage) over a wide current range. During major over-voltage conditions, the MOVs may generate heat so excessive that they will rupture or explode, thereby allowing the related electronic equipment to be destroyed.
Some related art surge suppression apparatus patents include:
(1) U.S. Pat. No. 6,055,147 to Jeffries et al. which teaches a plurality of MOVs being in series with a plurality of thermal fuses;
(2) U.S. Pat. No. 5,412,526 to Kapp et al. which teaches a plurality of MOVs being in series with a plurality of thermal fuses, and the plurality of MOVs being in parallel with a plurality of resistors;
(3) U.S. Pat. No. 6,040,971 to Martenson et al. which teaches a plurality of MOVs being in parallel with one another, an MOV being in parallel with a thermal fuse, and the thermal fuse being in series with a circuit protection device having an over-voltage trip device;
(4) U.S. Pat. No. 5,388,021 to Stahl which teaches a plurality of MOVs being in parallel with a plurality of resistors and a plurality of inductors, and a plurality of MOVs being in parallel with another;
(5) U.S. Pat. No. 5,675,468 to Chang which teaches two input terminals, two fuses in series respectively, a triac, an MOV in series between one fuse and an output terminal, a static potential limiter between the triac and ground;
(6) U.S. Pat. No. 4,901,183 to Lee which teaches two MOVs being in parallel with one another, two thermal fuses being in parallel with one another, a capacitor being in parallel with an MOV, two capacitors being in parallel with one another, and two inductors being in parallel with one another;
(7) U.S. Pat. No. 6,038,117 to Dullni et al. which teaches pairs of MOVs being in parallel, a plurality of such pairs being in series with one another and in series with a relay;
(8) U.S. Pat. No. 5,534,769 to Ishii which teaches resistor-capacitor combinations; and
(9) U.S. Pat. No. 5,808,850 to Carpenter, Jr. which teaches a three-phase device having quadruplets of parallel MOVs, each quadruplet being in series with a thermal fuse and an inductor, and bulbs for an indicator light for line to ground only.
A typical problem not addressed by these related art devices is that if an electrician accidentally reverses the line wire and the neutral wire, creating an extremely hazardous situation at the wall receptacle, the device may not adequately succeed in disconnecting the circuit to a peripheral device. In addition, the related art patents maintain an inordinately high component count to achieve their purpose of surge suppression. Therefore, a need exists for a circuit apparatus and a method which prevent (1) the hazardous condition occurring at the wall receptacle, arising from accidental reversal of the line wire with the neutral wire, from adversely affecting at least one peripheral device in the line, and (2) the overheating and destruction of MOVs in an over-voltage protection circuit by providing an efficient circuit configuration (i.e., the minimum number of components for device size reduction) which assures a rapid simultaneous disconnect of both the hot line and the neutral line.
An object of the present invention is to provide an apparatus and a method which prevent the overheating and destruction of MOVs in an over-voltage protection circuit by providing an efficient circuit configuration (i.e., the minimum number of components for device size reduction) which assures a rapid simultaneous disconnect of both the hot line and neutral power lines, and responds to the overheating of MOVs in an over-voltage protection circuit by opening the circuit to simultaneously disconnect both the hot line and the neutral line feeding a load when the over-voltage event occurs in the line-to-neutral (L-N) pairing.
Another object of the present invention is to provide an apparatus and a method which prevent the hazardous condition occurring at the wall receptacle which arises from accidental reversal of the line wire with the neutral wire from adversely affecting at least one peripheral device in the line, by detecting such reversal upon connection of the apparatus to the wall receptacle and alerting the user to the hazardous condition by appropriate illumination of warning lights or sounding of an audible alarm, or both. The user can then effect a correction before proceding.
Accordingly, the present invention provides a single-phase over-voltage protection circuit apparatus for all AC power lines and combinations thereof. In particular, the present invention provides an apparatus and a method which (1) indicates the hazardous condition occurring at the wall receptacle arising from accidental reversal of the line wire with the neutral wire from adversely affecting at least one peripheral device in the line by alerting the user to the unsafe condition, and (2) the overheating and destruction of MOVs in an over-voltage protection circuit by providing an efficient circuit configuration which assures a rapid simultaneous disconnect of both the hot line and neutral line. The present invention achieves this by opening the circuit to disconnect either or both of the hot line and neutral line feeding a load when the over-voltage event occurs in the hot line to neutral line (L-N) pairing.
By example, in a single phase AC power system, the hot line, the neutral line, and ground line (G) are all paths vulnerable to destructive high voltages. The present invention protects these paths by providing protection devices in the form of metal oxide varistors (MOVs) in parallel for each pair of wires (i.e., hot line to neutral, hot line to ground, and neutral to ground). The apparatus additionally comprises in-line fuses that disconnect both the hot line as well as the neutral line from the load. The MOV devices are activated by an over-voltage condition and absorb the extra and potentially damaging power surges and over-voltages; when the capacity of a MOV is exceeded, the MOV will generate excessive surface heat to transfer and current to flow to a thermal fuse, which then opens the fuse to protect the load.
Accordingly, in the event of a sufficiently high voltage (i.e., VL-N greater than 120V to 130V), from hot line-to-neutral or hot line-to-ground, causing excessive heating of the MOV protection devices, the hot line and neutral line will be disconnected from the user""s connected equipment. This disconnect is facilitated by the use of an open circuit condition of the in-line special thermal fuses, a non-resettable disconnect condition. The blown thermal fuses must be replaced to restore the circuit.
The current through the line may be denoted by IL. The current through an MOV, IMOV, must remain less than the rated current for the MOV, IMOV-rated, in normal operation. During an over-voltage event where the current through the MOV exceeds its current rating (i.e., IMOV greater than IMOV-rated), thereby emitting high surface temperature and effecting a current through the thermal fuse which exceeds its current rating (IL=ITF, ITF greater than ITF-rated), such thermal fuse will open, thereby preventing voltage application to the load (See FIG. 1).
In all the foregoing protection modes (i.e., L-N, L-G, N-G), a warning feature may be provided which indicates that a protection device (e.g., an MOV) has absorbed an excessive energy, thereby opening a thermal fuse or circuit breaker which effectively opens the path to any protection device and to the load. The warning feature may continue to alert the user even if power is no longer applied to the equipment. This warning feature alerts the user of the otherwise potentially destructive event. Upon so alerting the user, the apparatus should be unplugged from the wall outlet and the thermal fuse(s) replaced or circuit breaker reset. This warning feature may be audio and/or visual in nature.
Other features of the present invention are disclosed, or are apparent in the section entitled xe2x80x9cDETAILED DESCRIPTION OF THE INVENTION.xe2x80x9d